γ-Aminobutyric acid (GABA) is one of the major inhibitory amino acid transmitters in the mammalian central nervous system (CNS) and acts by binding to specific receptors in the plasmamembrane of both pre- and postsynoptic neurons. The binding of GABA to specific receptors causes the opening of ion channels in the cell membrane which allows either the flow of negatively-charged chloride ions into the cell or positively-charged potassium ions out of the cell. This typically results in a negative change in the transmembrane potential which usually causes hyperpolarisation.
There were once thought to be three types of receptors for GABA in the mammalian CNS, designated A, B, and C. GABAA and GABAC receptors are GABA-gated chloride ion-conducting channels while the GABA B receptor is a member of the G-protein receptor superfamily. GABAA and GABAC receptors were initially distinguished from one another by their sensitivity to the ligand bicuculline with the former being antagonised by it while the latter were insensitive. However, it has become increasingly clear since the mid-1990 s that the GABAA and GABAC receptors are simply variants of the same GABA-gated chloride channel. Therefore, these receptors are now denoted by the “GABAA” receptor designation. While varieties of the GABAA receptor are found all over the CNS, the GABAC receptors (GABAA variant also now defined variously as GABAAOr) are primarily found in the retina.
The GABAA receptor is a member of the Cys-loop ligand-gated ion channel superfamily which also includes the glycine, 5-hydroxytryptamine (5-HT, serotonin), and nicotinic acetylcholine receptors. Receptors of this superfamily consist of pentamers of homologous subunits arranged around a central ion-conducting channel. There are 19 different subunit genes—not including alternatively-spliced variants such as the short (S) and long (L) forms of the 1-6, γ1-3, αγ2 subunit—divided into eight subunit classes: β1-3, θ, ρ1-2, δ, π, ε (listed according to sequence relatedness). It is presumed that these subunits all arose as a result of gene duplications of an original sequence. Within a class of subunits there is approximately 70% sequence identity, and between subunit classes approximately 30% sequence identity. The majority of GABAA receptor subtypes in the mammalian brain contain at least one α, β, and γ subunit. Most GABAA receptors consist of assemblies of these three subunit classes. The most abundantly expressed isoform of the GABAA receptor in the mammalian brain is composed of α1, β2, and γ2, and the likely stoichiometry is two α, two β and one γ subunit arranged around the ion channel anti-clockwise γ-β-α-β-α as seen from the synaptic cleft. GABAA receptors of these subtypes are overwhelmingly numerically dominant in the CNS.
Each subunit of the GABAA receptor has a common structure consisting of a large amino-terminal portion, four transmembrane helices—designated transmembrane (TM) one to four, and a short, cytoplasmic loop toward the carboxy-terminus that is composed of the loop extending between TM3 and TM4. The receptor subunits are arranged pseudo-symmetrically so that the TM2 helix of each subunit lines the central pore. Recent models of the structure of the GABAA receptor have been based on the crystal structure of the related acetylcholine binding protein.
GABAA receptors can exist in at least three different conformations: open, closed, and desensitised. Activation of the GABAA receptor by GABA binding to the GABA site allows chloride ions to flow through the central pore and hyperpolarise the neuron, decreasing the probability that it will propagate an action potential. In this activity, the GABAA receptor does not differ from any other ligand-gated ion channel. However, up to 14 different ligand binding sites have been proposed to account for the modulation of GABA. Thus among neurotransmitter receptors, GABAA receptors are unique in view of the fact that their are a large number of ligands that can bind and allosterically modulate their function.
Binding of ligands to the GABAA receptor can alter the conformation of the GABAA receptor in such a way as to enhance or diminish the chloride flux in response to GABA binding. Some anesthetics (e.g. etomidate and pentobarbitone) both enhance chloride flow in response to GABA binding as well as activating it directly in the absence of GABA. Other ligands, such as cage convulsants of the picrotoxin type, bind within the central pore of the receptor thus, occluding the channel and preventing chloride flow, an effect which occurs no matter what other ligand subsequently binds. Hence, the neurophysiological effects of GABA result from a conformational change that occurs upon binding of GABA to the GABAA receptor.
The most widely studied and characterised class of allosteric modulators of the GABA-GABAA receptor complex are a class of compounds known as benzodiazepines (an example of which is diazepam a 1,4-benzodiazepine, commonly known as Valium®) which interact with the benzodiazepine (BZ)-site on the GABAA receptor. Possession of a γ subunit and a particular type of α subunit (1, 2, 3, or 5) is required to confer sensitivity to this class of compounds.

Classical benzodiazepines do not directly open the ion channel, rather they allosterically modify the GABAA receptor upon binding, potentiating the effect of GABA binding when there is a submaximal concentration of GABA present and thereby increasing hyperpolarizing responses and neuronal inhibition. Benzodiazepines produce systemic effects that include sedation, amnesia, muscle relaxation, and anxiolysis. Hence, these compounds are widely used as anxiolytics, sedative-hypnotics, muscle relaxants, and anti-convulsants. Benzodiazepines were the most widely prescribed class of drugs during the 1970s and, as a group, have one of the largest therapeutic indexes. Although the GABAA binding site is called the benzodiazepine site, drugs of other types can also bind and allosterically modify the receptor at that site. These include drugs with β-carboline, imidazopyridine, and triazolopyridazine structures. It is believed that compounds acting as BZ agonists at α1βγ2, α2βγ2 or α3βγ2 subtypes will possess desirable anxiolytic activity. Such modulators of the BZ binding site of GABAA are known herein as “GABAA receptor agonists”.
However, while the 1,4-benzodiazepines are an effective class of anxiolytics they possess the often unwanted side-effect of sedation. It is postulated that at least some of the unwanted sedation experienced by known anxiolytic drugs which act through the BZ binding site is mediated through GABAA receptors containing the α1-subunit. This has been determined primarily from the effects displayed by the well studied hypnotic agents Alpidem and Zolpidem which are α1-selective GABAA receptor agonists.

Thus in order to minimise the sedation effect, while still maintaining effective anxiolytic activity recent research has turned to finding GABAA receptor agonists which interact more favourably with the α2 and/or α3 subunit than with α1.